STRAINERS 153
The earlier forms of strainers were flat boxes with perforated metal plates
forming the bottom, the perforations being in the form of narrow slits about
2 to 3 inches long. The slits vary in width according to the form of pulp which
has to be strained, and the 'cuts', as they are called, are known by numbers,
such as '3' cut, or by their width in thousandths of an inch. The cuts are spaced
about £ inch apart, or far enough to prevent long fibres from getting their
ends through two different slits. Flat strainers of a modified and improved
type are used extensively in America for straining all kinds of stock, and they
still have adherents in this country, especially among paper-makers dealing
with strong rag stock. These strainers consist of a flat trough, the bottom
of which is formed of the strainer plates with slits. Underneath is another
trough, the bottom consisting of a flexible metal diaphragm, attached under-
neath to rods which in turn are in contact with cams on a rapidly revolving
shaft. These cams lift and drop the rods, and this causes the metal diaphragms
to oscillate up and down, thus causing an alternate sucking action through
the strainer plates, causing a partial vacuum on the downward stroke and a
reflex action on the upward stroke.
The suction downwards draws fibres and water through the slits, but it also
causes fibres to be drawn over the narrow slits and so clogs them up. The
'reflex* or upward flow on the upward stroke of the diaphragm pushes them
away, mixes them up with water, and some are thus ready to flow through
the slits at the next downward movement of the diaphragm.
If there were no 'reflex action*, the strainer would quickly become
completely clogged up with a coating of stuff, just as in the case of the
revolving drum or mould on a 'mould' machine. This 'reflex action' is
an essential feature of all strainers, and many different methods are used to
promote it.
The slits are always narrowest at the side on which the fibres enter, and
they open out towards the opposite side. This prevents them becoming clogged
up by congregations of fibres trying to squeeze through.
The passage of the fibres and water through the slits is brought about by
two chief causes: First, gravity, or the weight of the water in which the fibres
are suspended; secondly, by the sucking action of the diaphragm.
The older type of jog strainers are operated by ratchet wheels, the teeth
of which engage and lift rods fixed to arms projecting from the sides of the
vat. The opposite side of the vat is hinged, so that the vat moves freely up
and down. The action is, therefore, actually a tipping motion. These strainers
are still much in evidence, as they are to be found in most mills, being used
in conjunction with the more recent revolving strainers as 'back knotters* or
auxiliary strainers, to which we shall refer in greater detail kter on.